This invention relates to a track-seeking apparatus for an optical disk drive, more particularly to a track-seeking apparatus capable of high-speed seeking as required, for example, in an optical disk drive used by a computer.
Optical disk are used for storing and retrieving a variety of information, including audio and video information and, more recently, computer data. FIG. 1 shows a schematic representation of an optical disk. The disk 1 has a plurality of tracks 2 on which the information is recorded. The tracks may have the form of concentric circles as illustrated in the drawings, or they may form a single continuous spiral, in which case a track consists of one 360-degree length (one complete turn) of the spiral.
Each track is identified by a unique number called the track address. If N is the number of tracks on the disk, the tracks are customarily numbered from 1, which is the outermost track, to N, which is the innermost track. The track address is recorded in an ID (identifier) field in the track. When information is stored on or retrieved from the disk 2, the track address of the desired track is specified, causing a track-seeking apparatus to seek this track so that it can be written into or read.
FIG. 2 is a block diagram showing the main components of a track-seeking apparatus in an optical disk drive. These include a motor 30; a light source 40; a pick-up unit 50; a driving means 60; an address reader 80; an input unit 90; and a control unit 100. The motor 30 rotates the disk at a constant rate. The light source 40 produces a light beam 41. The pick-up unit 50, which is mounted on the driving means 60 so that it can be moved back and forth in the direction of the arrow 70, focuses the light beam 41 onto the disk 1, detects the reflected light, and converts the detected light to one or more electrical pick-up signals S50 containing the information recorded on the disk, which it furnishes to the address reader 80 and the control unit 100. The pick-up signals S50 are also supplied to external equipment for reproducing the information on the disk. The address reader 80 extracts track address information from one of the pick-up signals S50 and sends this track address information to the input unit 90. The input unit 90 receives this track address information and external commands, such as the address of the track to be accessed, and exchanges control signals with the control unit 100. The control unit 100 also receives the pick-up signals S50; the function of the control unit 100 is to count the number of tracks moved or crossed by the pick-up unit 50 and send a drive control signal S100 to the driving means 60. The driving means 60 moves the pick-up unit 50 in accordance with this drive control signal S100.
The track-seeking apparatus in FIG. 2 operates as follows. When the input unit 90 receives a command to access a specified target track, first it obtains from the address reader 80 the address of the current track and performs a subtraction operation to determine the difference between the current track address and the target track address. The magnitude of this difference is the number of tracks the pick-up unit 50 must move or cross to reach to target track. The sign of the difference indicates the direction in which the pick-up unit 50 must move toward the center or toward the periphery of the disk 1. The input unit 90 sets the magnitude of the difference in a counter in the control unit 100 as an initial remaining track count, and sends the control unit 100 a direction signal indicating the desired direction of movement. From the remaining track count and direction signal, the control unit 100 generates a drive control signal S100 which causes the driving means 60 to move the pick-up unit 50 in the desired direction. From the pick-up signals S50, the control unit 100 also determines when the light beam 41 crosses a track on the disk 1, and decrements the remaining track count by one for each track crossed. When the remaining track count reaches 0, the drive control signal S100 causes the driving means 60 to stop moving the pick-up unit 50, and the control unit 100 notifies the input unit 90 that the seek operation is completed. The input unit 90 then obtains the address of the current track from the address reader 80 again to check that the correct target track has been reached. If it has not, the seek operation is repeated.
A key factor in the seek operation described above is the way in which the control unit 100 detects track crossings. FIG. 3 illustrates a prior-art scheme for detecting track-crossings as employed, for example, in the track-seeking apparatus described in Japanese Patent Application Publication No. 48055/1985. As shown in waveform (a) in FIG. 3, when the light beam 41 is directed onto a track, the pick-up signal S50 received by the control unit 100 contains high-frequency components reflecting the information recorded in the track, but when the light beam 41 is between tracks, the pick-up signal S50 is substantially flat. The control unit 100 contains an envelope detector which detects the envelope of the pick-up signal S50, thus generating waveform (b) in FIG. 3. A Schmitt trigger circuit in the control unit 100 converts the waveform (b) to a pulse waveform (c) which is supplied as input to the counter in the control unit 100, with one pulse corresponding to one track.
A problem with this prior-art scheme for detecting track crossings is that it does not work when the pick-up unit 50 moves so rapidly that the rate of track crossings approaches the frequency of the information components in the pick-up signal S50, for then the envelope of the pick-up signal S50 can no longer be detected correctly and tracks are miscounted. Accordingly, the motion of the pick-up unit 50 must be limited to a comparatively low velocity. This low velocity is adequate for disks containing audio and video information, because such information is usually accessed sequentially and the pick-up unit 50 rarely has to move between widely separated tracks. It is inadequate, however, for disks containing computer data, because computer data tend to be randomly located on the disk, requiring the pick-up unit 50 to move frequently between widely separated tracks.